a 10-inch greaseproof cake board; fresh flowers; floral tape; a piping bag
For the cake: Preheat the oven to 350 degrees F. Line three 8-inch round pans or four 7-inch cake pans with parchment rounds, then grease with nonstick cooking spray.
Mix together the flour, granulated sugar, baking powder and salt in a stand mixer fitted with the whisk attachment until fully combined. Mix chunks of butter slowly into the dry mixture on low speed until no large chunks of butter remain and the mixture becomes crumbly. Pour in the egg whites and mix on low speed until just incorporated. Add the sour cream, almond extract, vanilla extract, and oil and mix until fully incorporated. Scrape down the sides of the bowl with a rubber spatula, then beat on low speed for a few more seconds.
Divide the batter evenly among the prepared cake pans.
Bake until a toothpick inserted in the center of a cake layer comes out clean, 33 to 35 minutes. Allow the pans to cool for 10 minutes, then run an offset spatula around the perimeters of the pans to separate the cakes from the pans. Place the cake layers in their pans in the freezer until fully cooled, 45 minutes to 1 hour. Carefully flip the pans and remove the layers from the pans. Use a serrated knife to level the tops of the layers.
For the almond buttercream: Meanwhile, beat the butter in a stand mixer fitted with the paddle attachment on medium speed until smooth, about 30 seconds. Turn to low speed and slowly add the powdered sugar, 1 cup at a time, alternating with small splashes of cream, until fully mixed. Add the almond extract, vanilla extract and salt and beat on low until the ingredients are fully incorporated and the desired consistency is reached. (If the frosting is too thick, add additional cream 1 teaspoon at a time. If the frosting is too thin, add more powdered sugar 1/4 cup at a time.) Add the gel food coloring, if using, and beat on low speed until it reaches the desired color.
Stack a cake layer on a greaseproof cake board using a dab of frosting to help it stick. Frost the top of the cake layer, then continue to stack the layers, adding a thin layer of buttercream between each layer. Apply a thin coat of frosting around the sides and on top of the cake to fully cover the cake layers. Smooth using a bench scraper, then chill the cake until the frosting is firm to the touch, 20 minutes in the fridge or 5 minutes in the freezer. Add a second, thicker layer of frosting, and smooth using a bench scraper.
Trim the stems of each flower to 1-inch long. Wrap the flower stems with floral tape, pinching and twisting at the bottoms to prevent fluids from the stems from leaking into the cake. Place the remaining buttercream in a piping bag and add a thick layer of frosting on top of the cake. Place the flowers into the frosting until the entire top of the cake is covered with flowers.